Recently, optical communication technology using visible light has attracted attention. Especially along with the rapid proliferation of lightings using lighting emitting devices such as Light Emitting Diodes (LEDs), extensive research has been conducted to realize a very convenient communication environment (a visible light communication system) utilizing indoor and outdoor lighting devices. Considering influence on human bodies and medical equipment, LED is the most promising candidate as a lighting device for optical communication. However, the transmission rate of data for optical communication depends on the response speed of a light emitting device or a driving circuit. Thus Laser Diode (LD) or Super Luminescent Diode (SLD) that has a higher response speed than the LED is a promising candidate, when high data rates are needed.
In addition, techniques for using Red, Green, Blue (RGB) LEDs and stably transmitting a large amount of data in a signal emitted by an LED have been proposed. These techniques include a multi-chromaticity transmission scheme. When RGB LEDs are used, the intensity of light emitted from each LED can be appropriately controlled, to thereby control the chromaticity of light observed by a receiver. According to the multi-chromaticity transmission scheme, a plurality of symbol points are set at chromaticity coordinates on a color system and data having a plurality of bits is mapped to a symbol point, for transmission. When the data is mapped to the symbol point, the intensity of light emitted from each LED is controlled to light having the chromaticity corresponding to the symbol point, as received at the receiver.
The intensity of light emitted from each LED can be controlled by controlling the magnitude of current pulses applied to the LED. The problem herein is non-linearity inherent to the light emission characteristics of the LED. In most cases, the light intensity of an LED increases non-linearly as the magnitude of current pulses increases (see FIG. 4). If light emission is controlled, ignoring the non-linearity of the LED, the chromaticity of light observed by the receiver does not match a symbol point at chromaticity coordinates. To prevent this disparity, the disparity needs to be compensated for by controlling a complex current, taking into account the non-linearity of the LED or predicting the disparity from the symbol point.
No method for compensating for the disparity of chromaticity has been specified in a visible light communication system. However, methods for compensating for non-linear distortion attributed to the non-linear characteristics of a transmission circuit in a wireless communication system are disclosed, for example, in Japan Laid-Open Patent No. 2001-203609 and Japan Laid-Open Patent No. 2001-295766. Japan Laid-Open Patent No. 2001-203609 discloses the configuration of a wireless communication system in which a receiver estimates an amplitude distortion resulting from applying a linear modulated signal to a non-linear circuit and compensates for the amplitude distortion of a received signal based on the estimation result. Japan Laid-Open Patent No. 2001-295766 discloses a structure that calculates an initial non-linear distortion coefficient (a deviation from a symbol point) by a training operation and compensates for the non-linear distortion of a received signal using the initial non-linear distortion compensation coefficient.